Romans dated their years according to who the consuls were in a particular year, i.e. referring to the year “when Gaius and Sulpicius were consuls”. In the later Empire, when the emperor was both deus et domine (lord and master) and ruled virtually by imperial decree, consuls were appointed by the emperor, and it was still regarded as a signal honour, in view of the fact that your name would thereby be guaranteed to live forever, the Empire being thought to be eternal. Nevertheless, the Romans also referred to a particular year in terms of the traditional date that Rome was considered to have been founded, i.e. 753 BCE. This was in all probability the result of contemporary historians who wanted a more convenient method when dating certain occurrences.
Utilising the tradtional founding date of Rome, a year would be expressed in terms of the number of years since the founding of Rome, i.e. Scaevola was consul in 623 A.U.C. (that is, in the 623rd year ab urbe condita = since the founding of the city).1 Dates referring to a year only were fairly simple things to express in Latin. It was a substantially more complicated matter to refer to specific days, though.
Originally Romans had only ten months in a year, beginning in March and ending in February. The Roman year was finally divided into twelve months, corresponding to our modern calender. The twelve months of the year were:
Ianuarius named after the god of doorways, the two-headed Ianus, looking in both directions to see who was approaching and who was leaving. This was traditionally the month in which the new consuls entered into office, therefore the idea of the doorway, and a new year (politically). It later became the first month for reasons of convenience, the new political year being a good point to start everything.
Februarius, for the religious feast of purification, held on 15 February annually, and called the Februa. This was originally the last month, and purification was necessary so as to start the new year with a clean slate (= tabula rasa) so to speak, presided over by the new consuls who traditionally took office the month before.
Martius, after Mars, the god of war, originally the first month of the year, probably coinciding with spring.
Aprilis, of which the etymology is unsure, some believing it had somethng to do with the opening of buds, from the Latin aperire, to open.
Maius, after Maia, the daughter of Atlas, and mother of Mercury by Jupiter, but why her we are not sure.
Iunius, after Juno, wife of Jupiter. (Had to give the boss’s wife some form of recognition.)
Iulius, after Julius Caesar of course (previously Quinctilius (quinque = five), the fifth month after March, which was originally the first month of the year).
Augustus, after Augustus, of course. If Caesar had a month named for him, Augustus had to have one too (previously Sextilis, the original sixth (sex = six) month of the year).
September, = the seventh (septem = seven) month of the original year, now ninth after adding Iulius and Augustus to keep those two gods (both these splendid chaps were deified) happy.
October, the original eighth month (octo = eight), now tenth for the same reason September is ninth.
November, the original ninth month (novem = nine).
December, the original tenth month (decem = ten).
Unlike our modern calender, the Romans referred to a specific day by linking it to certain core days that ocurred in every month. Thus every first of the month was described as being the calends, so, the first of January would be kalendis Ianuarii. This is where it stopped being simple, and became increasingly complicated. The Romans liked adding and subtracting, and this is also evident in their method of counting. Forty, for instance, is written as being ten less than fifty, i.e. XL, while sixty is ten more than fifty, i.e. LX, or eighty nine being eleven less than one hundred, i.e. XIC. They did the same with their dates.
After the kalendis, came the nonae, being the ninth day before the ides of the particular month, therefore falling on the 5th of every month, except for March, May, July and October, when it fell on the 7th of the month. The following day referred to was the ides, the idus, falling on the 13th of every month, except (again) for March, May, July and October, when it fell on the 15th. Thus, 5 June would be nonis Iunii, while 7 March would be nonis Martii. The idibus Martii, the ides of March would be the 15th of the month. (February, having only twenty eight days had its own problems, but these were fortunately not serious.)
All other days would be described in relation to the kalendis, nonae or idus, by stating how many days before this particular day it would fall. So, the 9th of March would be described as ante diem septem (VII) idus Martium, i.e. seven days before the ides of March. Similarly the 9th of February would be described as ante diem quinquem (V) idus Februarii, the fifth day before the ides of February (February having only 28 days). After the ides of the month, the day would be described in terms of how many days it was before the kalendis of the next month, thus, 17 January would be described as ante diem XVI kalendas Februarii, being sixteen days before the first day of February.
In the last instance, the day before the kalendis, nonae or idus was not counted as one, but referred to as pridie, i.e. the day before the nonae or idus etc. Thus 30 November would be referred to as pridie kalendas Decembris, and 4 June would be describes as pridie nonas Iunii.
Complicated? Yes, but it must be remembered that the Roman civilisation was an extremely complex conglomeration of people, systems and above all religion and tradition, that permeated every level of society. Nothing was simple, except the concept of Roman honour. You simply killed yourself rather than suffer ingnominy or dishonour.
1 Romans recognised only one city: Rome. The rest were, well, just places elsewhere on the map of the world.